1. Field of the Invention
The present invention relates to image acquisition apparatuses and image acquisition methods that use optical coherence tomography.
2. Description of the Related Art
In recent years, image acquisition apparatuses that use optical coherence tomography (referred to as “OCT” hereinafter) have been used in the ophthalmologic field for acquiring fundus tomographic images.
Accuracy in diagnosis is significantly affected by image distortion (motion artifact) caused by biological motion (eye motion in particular) occurring during a fundus image acquisition operation by OCT. A typical eye motion involves a three-dimensional involuntary eye movement of about 100 μm per second in the planar direction (referred to as “horizontal direction” hereinafter) as well as the depth direction (“referred to as “vertical direction” hereinafter) of a fundus.
OCT mainly includes two methods, namely, time domain OCT (TD-OCT) and Fourier domain OCT (FD-OCT). In TD-OCT, about one second is required for acquiring a tomographic image (a two-dimensional image constituted by a one-dimensional image in the horizontal direction and a one-dimensional image in the vertical direction) by B-scanning. Therefore, in order to acquire about 100 three-dimensional images by B scanning, there is a problem in that the image acquisition takes too much time relative to the eye movement. On the other hand, in FD-OCT, high-speed image acquisition (in which a three-dimensional fundus image can be acquired in about one to three seconds) that is ten times faster than TD-OCT is possible. FD-OCT includes spectral domain OCT (SD-OCT) and swept source OCT (SS-OCT).
Ophthalmologic OCT is desirably applied to screening in group examinations for the purpose of early detection of three major diseases (diabetic retinopathy, glaucoma, and age-related macular degeneration) that can possibly lead to blindness. In group examinations, it is essential to increase the number of examined people per unit time as much as possible. Therefore, it is important to reduce the number of retakes of images. However, in image acquisition using ophthalmologic OCT, if image distortion occurs as a result of eye movement or body movement, the image needs to be retaken again. In addition, since it is necessary to maintain the image quality as much as possible to prevent lowering of diagnostic quality, high-speed image acquisition is desired. Moreover, although collective image acquisition of a wide region including a macula and an optic disk is desired, an increase in an image acquisition region leads to a longer image acquisition time.
In light of this, in image acquisition using OCT, there is a method of efficiently acquiring image information of an area of diagnostic importance by setting the number of scan lines extending in the area of importance greater than that in other areas. FIG. 10C illustrates a grating that allows the center of an OCT image acquisition region 2701 to be scanned more than other regions when an area of importance is located in the center. Each line schematically represents a scan line 2702 that corresponds to a single main scanning process. Furthermore, US Patent Application Publication No. 2007/0195269 discloses a method in which the grating is disposed in a polar coordinate system, the distance between scan lines is adjusted in a radial direction and a tangential direction of a circle, and an area of importance is weighted.